Circulating fluidized bed combustion (CFBC) reactors are being utilized in an increasing number of applications including the generation of steam wherein fossil fuels such as coal are used as a fuel source. CFBC reactors typically utilize fuels having a lower energy content than are used in standard boiler furnaces. CFBC reactors are favored over conventional boiler systems because the combustion flue gas can be desulfurized economically within the combustion chamber before being emitted into the atmosphere.
In a typical process which employs a CFBC reactor for producing and recovering heat energy, a bed of solid particles is initially introduced into, and then maintained in the reactor in order to sustain a nearly uniform temperature throughout the combustion bed. The bed of solid particles adsorbs heat from the combusting fuel source and serves as a means of conveying and transmitting such heat to a plurality of adjoining cooling tubes containing a circulating fluid which surround the reactor combustion zone. Typically, the bed of solid particles comprises ash particles produced by combustion of a fuel source although the bed may additionally comprise sand or other materials introduced into the reactor to assist in the maintenance of the desired reactor temperature profile. A solid fuel and an oxygen-containing gas are introduced into the reactor and are combusted thereby releasing a substantial amount of heat and forming hot exhaust gases containing ash particles. The oxygen-containing gas, along with the gaseous products of combustion, fluidize the bed of particles within the combustion zone thereby providing intimate mixing of the combusting fuel and combustion particles.
A first portion of these particles impacts the walls of the reactor and typically falls downward to the bottom of the reactor simultaneously transmitting heat to the water or other fluid contained within the heat transfer tubes surrounding the combustion zone. A second portion of the particles is carried upward within the reactor along with the exhaust gases. The major fraction of the upward flowing stream of particles is captured in a cyclone or other particulate collection device and is typically returned to the combustion zone. The remaining fraction of particles which is not captured by the cyclone is carried by the exhaust gases into a convection section. Such particles are known as fly ash.
Fly ash must be separated from the flue gas prior to expulsion of the flue gas into the atmosphere. This separation step is typically accomplished by use of fabric filters or other suitable apparatus. For coals which produce large quantities of fly ash, the loss of such fly ash from the combustion zone causes the bed density within the combustion zone to drift below the operating range which is most effective for promoting heat transfer rendering a reactor temperature which is too high to conduct efficient removal of pollutants.
The heat released by the combusting fuel is quickly absorbed by the large quantity of solid particles due to the intimate mixing between the burning fuel and the solid particles returning to the bottom of the reactor. While the particles ultimately transfer the heat that they acquire to the walls of the reactor, under optimum conditions only a small temperature change in the reactor is experienced due to the large mass, and therefore, high heat capacity of the circulating particles.
In the case of CFBC reactors, sufficient heat transfer is dependent upon maintaining an adequate concentration of ash particles within the reactor which promotes mixing and heat transfer. When the average ash particle size is too small, ash tends to exit the reactor and cyclone causing a rise in bed temperature. Likewise, If the average ash particle size is too large, insufficient ash particles will be fluidized also causing a temperature fluctuation within the reactor.
Several approaches have been proposed for regulating particle bed density within CFBC reactors in order to minimize temperature fluctuations and enhance heat transfer between the heated flue gas containing ash particles and the cooling tubes surrounding the reactor. For example. U.S. Pat. No. 4.111,158 discloses a method and apparatus for carrying out an exothermic process in which a combustible solid is burned in a fluidized-bed reactor with primary and secondary streams of oxygen-containing combustion-sustaining gas supplied at different levels so that the primary gas acts as the fluidizing gas. The major portion of solid feed is introduced into the space below the secondary gas inlet which is maintained substantially free of internal fixtures and the effluent gases are separated from the entrained solids which are recycled to the bed. Solids withdrawn from the bed are cooled in a fluidized bed cooler and the temperature of the bed is maintained constant by the controlled recirculation of cooled solids thereto from the cooler. The gas heated in the cooler is fed to the bed as secondary gas.